@rseod Exit Emulationstation (either press F4 on the keyboard or use the menu to exit) and look at the message shown on the screen - it should show how much disc space you have available (%). If your free disc space is less than 5%, then you might have a problem and you need to free up some space.

...I think it was my fault...
I was thinking, that retropie shows me the usb files, when i plug in the stick...
But it look like, it shows alltime the files on the sd card ;-)
...so I have created a folder "retropie-mount" on the usb stick, let retropie copy all roms to it, and after that I deleted any files in "\retropie\roms"
Look like this solve my issue.

I think now I understand the differents between the directory "retropie" and "retropie-mount"
...if I want to import roms to the sd card, i put the files on the usb stick to "retropie" and if I like to use a external usb stick as storage for the roms, I use the "retropie-mount" directory on the usb stick...
Is this right?

@redneckmatt thanks. Should be fixed now in the development branch.

@worcesterthecat ctrl + a selects all files

Okay, it looks like I have it running pretty clean now. Thank you all so much. Next I'll try to figure out how to add Famicom CDS, Super Famicom, Virtual Boy and maybe Game & Watch directories/roms to my build. Time to start hunting through the threads [slaps and rubs hands together in eager excitement] :)